1. Field of the Inventiuon
The present invention concerns a method, as well as the equipment designed to carry-out such a method, to obtain the reproducibility of the retention times of the components of a mixture analyzed in an instrument of gas chromatographic analysis, when the capillary column used is replaced with another nominally equal column in the same equipment or in different but nominally equal equipments operating under the same conditions.
The problem which underlies the present invention arises from the observation that the retention times of a given component in a mixture to be analyzed by gas chromatography show values which vary from column to column and from equipment to equipment, even if the nominal features of columns and equipments are exactly the same. This causes a shift along the axis of the chromatogram times of the peak related to the concerned component versus the same peak obtained in an identical analysis in a different, while nominally equal column.
2. Description of the Prior Art
It is known that the result of a gas chromatographic analysis is a chromatogram formed by a sequence of peaks, each indicating the arrival of a component of the analyzed mixture at a detector placed downstream of the column after a so-called retention time tr, measured from the sample introduction into the column, and representing the time required to pass through the column.
The retention time tr obviously depends on the nature of the concerned component, on the nature and thickness of the stationary phase film present in the column and on the analytical procedures, particularly on the temperature program of the column and on the carrier gas pressures at the column inlet and outlet. The retention time tr is also affected, for the same component and under the same conditions, by the characteristics of the capillary column, particularly its length and diameter.
As a consequence, the variations in the retention times of the components of the same mixture, namely the peak shifts that are detected in equal analyses with different but nominally equal columns, depend on the different geometry of said columns.
In fact it was observed that nominally equal columns, i.e. having the same diameter and the same nominal length, actually have not negligible differences both in length and internal diameter, which obviously is not uniform and controllable on the entire extent of the column and varies from one column to the other.
These differences versus the nominal values create said peak shifts in identical analyses on different but nominally equal columns, in the same equipment or in different but nominally equal equipments. This shift involves the need, when replacing the column or changing the equipment, to change the parameters used for the recognition of peaks (peak tables), which means that it becomes necessary to modify the analytical method with waste of time, extra costs and failure to comply with GLP (Good Laboratory Practice) requirements.
To eliminate this drawback, it was already proposed to use a methodology named blocking of the retention time, which consists in the following procedure. A standard mixture is chosen and a reference analysis is run at a defined inlet pressure of carrier gas, identifying a reference compound and the relevant retention time. After the column replacement, analyses are run with the same mixture and at carrier gas inlet pressures equivalent to 80%, 90%, 100%, 110% and 120% respectively of the previously used inlet pressure, acquiring the retention times for the reference compound. Then a graph is plotted, indicating the retention times as a function of the inlet pressure and joining the points plotted with straight line segments. Then the original retention time is entered in the graph to obtain the new inlet pressure that must be maintained in the subsequent analyses to obtain the realignment of peaks.
This method, however, is rather time-consuming and complicated. Moreover, it allows the realignment of peaks only when these are close to that of the reference compound. But the most serious inconvenience is that this method requires all subsequent analyses to be carried out in the constant pressure analytical mode, whereas running analyses according to the carrier constant flow rate mode is more advantageous as far as detector response and total analytical time are concerned.
Assuming that, when the column is replaced or the equipment changed, the geometry of the new column is responsible of the peak shift, i.e. of the variations of retention times, the Applicant has resolved to apply a method that allows to automatically characterize each column used, keeping into consideration the actual values of length and internal diameter of same, and to use such characterization to obtain an automatic realignment of all peaks, without differences caused by their position on the chromatogram, whatever the analytical mode selected (feeding of carrier gas at constant or programmed flow rate, or at constant or programmed pressure).